By way of example, a particle source that produces an electron beam is used in a scanning electron microscope, the electron beam being focussed on the surface of an object to be examined, wherein the location of the focus, i.e. the location at which the electrons are incident on the object, is scanned step-by-step over the surface of the object by deflecting the electron beam. The focus remains at each location for a period of time that is sufficient to be able to detect a desired number of events that are triggered by electrons incident on the object. By way of example, the events can be secondary electrons, x-ray radiation or other types of events that are triggered by an electron incident on the object. The detected events that are assigned to the various locations of the object represent an electron-microscopic image of the object. In order to be able to record an image with a desired contrast, it is desirable to detect a sufficient number of events, for the purposes of which a minimum number of electrons is incident at each location on the object. This minimum number depends on the properties of the object, the type of detected event, the noise of detectors and other components, and further factors.
Therefore, within the scope of increasing a throughput when obtaining electron-microscopic images, it is desirable for the particle beam that is directed onto the object to have a beam current that is as high as possible. However, an increase of the beam current in a given scanning electron microscope leads to an increase in a diameter of the beam focus that can be produced at the surface of the object by way of the scanning electron microscope since the charged particles in the beam interact with one another. The resolution of the scanning electron microscope is restricted by the diameter of the beam focus. Consequently, within the scope of increasing the resolution, it is desirable, on the other hand, to reduce the beam current.
Consequently, in the case of a given scanning electron microscope, it is possible to increase the throughput to the detriment of the resolution or it is possible to increase the resolution to the detriment of the throughput.
In a typical scanning electron microscope, the maximum size of the beam current is determined by the size of the cross-sectional area of an aperture in an anode stop that defines the beam. Increasing the cross-sectional area of the aperture leads to an increase in the maximum possible beam current. Therefore, anode stops with differently large apertures are obtainable for the typical scanning electron microscope in order to be able to change the maximum beam current by interchanging the anode stops. However, interchanging the anode stops, which are arranged in a vacuum during the operation of the scanning electron microscope, is a complicated process involving much time. Therefore, use is often made of two scanning electron microscopes, possibly even of the same design, in laboratories, the scanning electron microscopes differing in view of the produced beam current in order to be able to alternatively examine specimens at a high beam current with a high throughput and at a low beam current with a high resolution. It is clear that the procurement of two scanning electron microscopes merely for the purpose of being able to carry out examinations with two different beam currents is considered to be unsatisfactory.